This invention relates to an integral heated probe for injection molding and a method of making the same.
In the past, it has been well known to make cartridge type heaters by running electrical heating wires through a heat transfer material in a casing and then swaging to compact the material, elminating voids which improves heat transfer. An illustrative example of this method and structure may be found in U.S. Pat. No. 2,831,951 which issued Apr. 22, 1958 to the Watlow Electric Manufacturing Company.
More recently, this concept has been adapted to provide a heated probe which extends into a hot runner passage to maintain the temperature of pressurized melt flowing longitudinally around it towards a gate. In such an application, heat transfer away from the electrical heating wires or element and to the outer casing are, of course, very important. If heat is not transferred quickly and adequately away all along the heating element it will develop a "hot spot" and burn out necessitating replacement. Furthermore, it is desirable that the outer casing be maintained at a substantially constant temperature along its length to improve the efficiency of heat transfer to the surrounding melt as well as to avoid any melt deterioration due to overheating, particularly with engineering or other difficulty to mold materials. Some of these probes are formed by inserting a swaged cylindrical cartridge type heater into a hollow outer torpedo body, but this has the disadvantage that the outer body must be made large enough to receive the cartridge heater which leaves a void or air space therebetween. While this has the advantage that burnt out cartridge heaters may be quickly replaced, it has the disadvantage that the air space acts as an insulator.
Attempts have been made to overcome this problem by inserting a conical shaped heater in a tapered hole, and even by providing a split sheath heater. However, while these are better, they still leave a considerable amount of insulative air space.
Another attempt to overcome this difficulty is shown in U.S. Pat. No. 3,970,821 which issued July 20, 1976 to Fast Heat Element Manufacturing Co., Inc. which discloses a heated probe which is made by swaging the torpedo body or casing directly over the magnesium oxide through which the heating element extends. While this does reduce the amount of insulative air space, it does not provide a satisfactory balance of heat transfer between the sides and the tip of the probe. There is considerably more heat transfer directly to the sides of the probe than the magnesium oxide powder which allows to the tip of the probe.